Re: pointers to member functions

From:
giulianodammando@libero.it
Newsgroups:
comp.lang.c++
Date:
18 Jul 2006 00:48:04 -0700
Message-ID:
<1153208884.352852.224100@m73g2000cwd.googlegroups.com>
Alf P. Steinbach wrote:

* giulianodammando@libero.it:

Thanks a lot Victor and Alf. In the meantime i've found the following
solution.
It works, but I don't know exactly how and why.


No-one can tell you unless you post the code.

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This is what i meant (should compile cleanly). Obviously this solution
is far from general
because one needs to recompile the C numerical integration library
library (in my case a few files), but I have not modified them
internally. So this serves my purpose. With a precompiled library I
think this method cannot be applied.

Best Regards
Giuliano

// abstract class
class func {
    public:
        func() {};
        virtual ~func() {};
        virtual const double operator() (const double x) = 0;
        /*virtual const array_1* operator() (const array_1 &vx) = 0;*/
};

// Wrapper for regular functions
template <class T>
class Regular_func : public func {
    public:
        typedef double (T::*func_ptr)(double);
        Regular_func() {}
        explicit Regular_func(T *t, func_ptr fun) { T_pointer=t; f = fun;}
        const double operator() (const double x) { return (T_pointer->*f)(x);
}
    /* const array_1* operator() (const array_1 &vx){ <-- commented (i use
external Array class)
            // returns array of values
            array_1 *y = new array_1;
            y->resize(vx.size());
            for(int i= vx.lbound(0); i < vx.ubound(0); i++){
                (*y)(i) = (T_pointer->*f)(vx(i));
            }
            return y;
        }*/
    private:
        T *T_pointer;
        func_ptr f;
};

/*
typedef double (*fptr)(double);

int integration_simple (fptr *f,
                            double a, double b,
                            double * result, double * abserr); */

int integration_simple (func *f,
                            double a, double b,
                            double * result, double * abserr);

int integration_simple (func *f,
                            double a, double b,
                            double * result, double * abserr)
{
    double result1,result2;
    double half = (b-a)/2;
    /* very simple trapezoidal rule */
    result1 = ((*f)(a)+(*f)(b))/2*(b-a);
    result2 = ((*f)(a)+(*f)(half))/2*half + ((*f)(half)+(*f)(b))/2*half;
    *abserr = result2-result1;
    *result = result2;
    return 0;
}

class CrossSection {
    public:
        CrossSection() : k(5.0) {};
        double Y(double x) { return k*x; }
        double integrate(double a,double b) {
                double result,abserr;
                Regular_func<CrossSection> f(this,&CrossSection::Y);
                integration_simple (&f,a, b,&result, &abserr);
                return result;
        }
    private:
        double k;
};

#include <iostream>
using std::cout;
using std::endl;
using std::cin;

int main() {

    double result;
    char dummy;

    CrossSection aCrossSection;

    result = aCrossSection.integrate(0.0,1.0);

    std::cout << result << std::endl;
   cin >> dummy;
}

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